Aspects of the invention relate generally to signal processing. More particularly, aspects of the invention relate to apparatus and method for signal processing a time code signal recorded on a disc for controlling a digital audio source.
Disc jockeys are able to manipulate audio files stored on a computer, while retaining the manual feel of traditional records, using emulation systems known as a digital vinyl system (“DVS”). A DVS utilizes a special recording medium known as a time code record, which has a time code signal circumferentially recorded thereon in lieu of music. The time code signal may be used to direct playback of the audio file (e.g., MP3, .wav, etc.) such that the playback mimics the motion of the time code record. Thus, the audio file sounds as if a traditional vinyl record is being played.
Conventional DVS time code signals typically are binary signals modulated onto a carrier wave. An accurate frequency reading of the carrier wave is essential for estimating the speed of the recording medium. Therefore amplitude modulation (“AM”) of the carrier wave is preferred over frequency modulation (“FM”). Phase locked loop (“PLL”) circuits are used to detect the frequency of the carrier wave in the time code signal, which represents the speed of the recording medium. A demodulated binary signal obtained from the time code signal is used to determine a position on the recording medium.
An input signal of constant amplitude is optimal for detecting frequency using a PLL, because an output of the PLL fluctuates when the amplitude of the input signal fluctuates. Thus, when a carrier signal of changing amplitude, such as an AM signal where the modulation is from the binary signal, is the time code signal, heavy noise reduction or smoothing typically is required to accurately determine the frequency of the signal. For example, low pass filtering may be used to smooth the output signal of the PLL and remove high frequency noise. Such filtering, in essence, has the effect of calculating a weighted moving average of the output signal of the PLL over time, which introduces a latency between the input signal and the output signal. The more aggressive the smoothing, the more samples over which the moving average is taken, and therefore the greater the latency. It is desired to minimize latency in DVS, because a reduced latency minimizes the perceived delay between movement of the record and the audible response. To accurately emulate the manipulation of a traditional vinyl record, there should be a minimum or no latency in audible response following movement of the record.
Therefore, there exists a need for reducing the time in which and increasing the accuracy with which speed and position may be determined from a time code signal retrieved from a rotating recording medium.